1. Field of the Invention
The present invention relates to a capacitive ultrasonic transducer, into which a silicon semiconductor substrate is processed using a silicon micromachining technique, and its production method, and to a capacitive ultrasonic probe comprising the capacitive ultrasonic transducer in an end portion of an insertion section inserted into a body cavity.
2. Description of the Related Art
An ultrasonic diagnosis of diagnosing by radiating an ultrasonic wave into a body cavity, and visualizing a state in a living body from its echo signal has spread. There is an ultrasonic endoscope as one of equipment and materials used for this ultrasonic diagnosis. In the ultrasonic endoscope, an ultrasonic transducer is mounted at an end of an insertion section inserted into a body cavity, and this transducer converts an electric signal into an ultrasonic wave, radiates into the body cavity and receives an ultrasonic wave reflected in the body cavity, and converts it into an electric signal.
Heretofore, although a ceramic piezoelectricity material PZT (lead zirconate titanate) has been used in an ultrasonic transducer as a piezoelectric element which converts an electric signal into an ultrasonic wave, a capacitive ultrasonic transducer (Capacitive Micromachined Ultrasonic Transducer (called a c-MUT)) into which a silicon semiconductor substrate is processed using a silicon micromachining technique attracts attention. This is one of elements generically named a micromachine (MEMS: Micro Electro-Mechanical System).
The MEMS element is formed as a fine structure member on a substrate, such as a silicon substrate or a glass substrate, and is an element made by combining a driver which outputs a mechanical drive force, a drive mechanism which drives the driver, a semiconductor integrated circuit which controls the drive mechanism, and the like electrically and further mechanically. A fundamental feature of the MEMS element is that the driver constructed as mechanical structure is incorporated into a part of the element, and drive of the driver is performed electrically by applying a Coulomb attraction between electrodes or the like.
Now, the capacitive ultrasonic transducer (c-MUT) is an element of two electrodes standing with facing each other, there is a cavity in between them, and when an AC signal superimposed on an DC bias is applied, a layer (membrane) including one electrode between them vibrates harmonically to generate an ultrasonic wave.
For example, a method of producing the c-MUT using wafer-boding technology is disclosed in prior art (Yongli Huang and four others, “Fabricating Capacitive Micro machined Ultrasonic Transducers With Wafer-Boding Technology”, JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 12, NO. 2, p. 128-p. 137, April, 2003. In this antecedent, the transducer is produced by forming a membrane and cavities on an SOI (Silicon On Insulator) wafer and a prime wafer respectively, and bonding those wafers using a silicon direct bonding technique in vacuum.
Ultrasonic transmission pressure P of a capacitive ultrasonic transducer is expressed as follows:P=−∈r×8.854e−12×W2/d2×V2 where
∈r: dielectric constant of material between electrodes
W2: electrode area
d: distance between electrodes
V: applied voltage
In addition, a center frequency fc is expressed as follows:fc=(π/2)×(tm/W2)(E/12ρ)1/2 where
tm: thickness of membrane
E: Young's modulus
ρ: density
Hence, although enlarging the electrode area W2 enlarges transmitted ultrasonic sound pressure, it causes decrease of the center frequency at the same timer and hence, it was extremely difficult to obtain high sound pressure in a high frequency domain.
In addition, heretofore, production of a capacitive ultrasonic transducer was not easy in an economic aspect. Furthermore, when using a capacitive ultrasonic transducer for an ultrasonic endoscope, it is necessary to radiate an ultrasonic wave with an acoustic impedance near an acoustic impedance of a tissue in a body cavity.
Recent years, although an ultrasonic transducer has been widely used for acoustic diagnosis and a piezoelectric element using piezoelectricity has been usually used for this ultrasonic transducer, the capacitive ultrasonic transducer mentioned above is proposed recently.
For example, a theoretical structural example of a capacitive ultrasonic transducer is disclosed in National Publication of International Patent Application No. 2004-503312. Since sensitivity of a capacitive ultrasonic transducer is low, it is desired to be able to make it more highly sensitive.
For this reason, a capacitive ultrasonic transducer with specific structure, that is, layered structure is disclosed in U.S. Pat. No. 6,558,330.
On the other hand, harmonic imaging diagnosis using a harmonic signal is becoming standard diagnostic modality because of a clear diagnostic image which is not obtained by conventional B mode diagnosis recently.
The harmonic imaging diagnosis is classified into (1) a tissue harmonic imaging method which splits by various methods harmonics which are influenced by nonlinearity of a living body tissue and superimposed on a fundamental ultrasonic wave when an ultrasonic wave spreads an inside of a tissue, and performs visualization using this signal, and (2) a contrast harmonic imaging method which injects contrast medium bubbles into an inside of a body, receives harmonics generated when the bubbles explode or resonate by radiation of a transmitted ultrasonic wave, splits the harmonics superimposed on a fundamental ultrasonic wave by various methods, and performs visualization using this signal.
It turns out that all of these have such a good S/N that cannot be obtained by a conventional B mode tomogram and that a diagnostic image with a satisfactory resolution is obtained, and they contribute to enhancement in diagnostic accuracy of medical diagnosis.
As for an ultrasonic transducer used for a conventional harmonic imaging diagnostic apparatus for an outside of a body, for example, the same ultrasonic transducer serving both for transmission and reception has been used also for fundamental wave transmission and harmonics reception. In addition, construction of receiving an echo of an ultrasonic pulse reflected from a living body tissue with an ultrasonic transducer provided separately from that for transmission is also possible.
Since a signal level of a harmonic signal is far small in comparison with a fundamental wave, it is necessary to remove efficiently a fundamental wave component in connection with degradation of a harmonic image. Therefore, harmonic component (in particular, second harmonic component) extraction technique which is widely known is used.
As ultrasonic transducers, besides a conventional piezoelectric ultrasonic transducer, the above-mentioned capacitive ultrasonic transducer into which a silicon semiconductor substrate is processed using a silicon micromachine technique attracts attention.
As for the capacitive ultrasonic transducer, it is said that, generally, in order to generate an ultrasonic wave, not only a high frequency pulse signal, but also a DC bias voltage is required at the time of both of reception and transmission. In short, it is performed to generate a signal that the high frequency pulse signal is superimposed on the DC bias voltage, to apply it to the capacitive ultrasonic transducer, and to transmit and receive the ultrasonic wave by it.
By the way, since the capacitive ultrasonic transducer conventionally having been proposed has an ultrathin membrane thickness, it reflects acoustic impedance of a cavity, and hence, it is suitable for air ultrasonic waves.
A capacitive ultrasonic probe apparatus aiming at use outside a body is disclosed in the above-mentioned National Publication of International Patent Application No. 2004-503312.
In order to use a harmonic imaging technique, an ultrasonic transducer with a wide band characteristic is necessary, but since the capacitive ultrasonic transducer has a wide band characteristic, it is suitable for harmonic imaging diagnosis.
In addition, in the case of a conventional capacitive ultrasonic transducer, since intensity of an ultrasonic beam is small, many capacitive ultrasonic transducer elements are used and ultrasonic beams transmitted by these are focused electronically.